Magnetic recording medium

ABSTRACT

A magnetic recording medium is disclosed, comprising a nonmagnetic support having thereon plural magnetic layers comprising ferromagnetic particles dispersed in a binder, said ferromagnetic particles consisting of an iron oxide or a cobalt-containing iron oxide, said plural magnetic layers comprising at least an upper magnetic layer and a lower magnetic layer, wherein said upper magnetic layer has a coercive force (Hc) of from 650 to 1000 Oe, a crystallite size of the ferromagnetic particles contained in said upper magnetic layer is less than 500 Å as measured by a X-ray diffraction method, an average length in the long axis of the ferromagnetic particles contained in said upper magnetic layer is from 0.1 microns to less than 0.25 microns as measured by a transmission type electronmicroscope, said lower magnetic layer has a coercive force (Hc) of from 0.6 to 1.0 times that of said upper magnetic layer, a crystallite size of the ferromagnetic particles contained in said lower magnetic layer is larger than that of said upper magnetic layer and is in the range of 500 Å or less, and an average length in the long axis of the ferromagnetic particles contained in said lower magnetic layer is larger than that of said upper magnetic layer and is in the range of from more than 0.1 microns to less than 0.25 microns.

FIELD OF THE INVENTION

The present invention relates to a magnetic recording medium comprisinga nonmagnetic support and a magnetic layer, and more particularly itrelates to a magnetic recording medium comprising plural magneticlayers.

BACKGROUND OF THE INVENTION

Recently with improvements in magnetic recording, there has been agrowing demand for a high image quality and a high audio quality. Tomeet such demand, ferromagnetic particles are pulverized and magneticflux density of the magnetic recording medium is increased. Also, anincreased consumption of a magnetic recording medium may require that itis inexpensive. A magnetic recording medium comprising plural magneticlayers have been proposed to achieve the foregoing requirements, becausean upper magnetic layer can provide characteristics for a high imagequality, and a lower magnetic layer can provide characteristics for ahigh audio quality, thereby causing suitable ferromagnetic particles foreach to be used. Further, the use of plural magnetic layers results inreduced costs because suitable materials can be used depending ondesired characteristics of each of the plural layers. In order to attaina high image quality and a high audio quality, it is required toincrease electromagnetic characteristics, particularly Rf output, todecrease noise, and to increase S/N.

To this end, it is required to increase the number of ferromagneticparticles per volume by making ferromagnetic particles small. However,even if the specific surface area of ferromagnetic particles isincreased to increase the number of ferromagnetic particles per volume,it is difficult to attain the above object because holes are formed inthe surface of ferromagnetic particles so that the size of ferromagneticparticles is not substantially reduced, and the ratio of the long axisto the short axis of ferromagnetic particles is reduced.

Namely, there are problems that when holes are formed in the surface offerromagnetic particles, the loss of magnetic ratio increases, and whenthe long axis/short axis ratios are reduced, the orientation property offerromagnetic particles is deteriorated at the time of the manufactureof a magnetic recording medium.

When ferromagnetic particles are made small, there is a further problemthat the transfer characteristics are deteriorated due to the increaseof components with a low coercive force, thus making the ferromagneticparticles impractical.

To overcome the above problems, JP-A-63-187419 (The term "JP-A" as usedherein means an "unexamined published Japanese patent application")proposes plural magnetic layers in which an average length in the longaxis of ferromagnetic particles contained in an uppermost magnetic layeris less than 0.25 microns, a crystallite size of the ferromagneticparticles contained in the uppermost layer is less than 300 Å asmeasured by X-ray diffraction, an average length in the long axis offerromagnetic particles contained in magnetic layers other than theuppermost layer is 0.25 microns or more, and a crystallite size of theferromagnetic particles contained in the magnetic layers other than theuppermost layer is 300 Å A or more.

The present inventors have found that when such plural magnetic layersare used in a video tape of VHS or beta type, Hc (i.e., coercive force)of an upper magnetic layer is preferably 650 to 1000 Oe, and Hc of alower magnetic layer is preferably 0.6 to 1.0 times that of the uppermagnetic layer.

However, it has been found that when ferromagnetic particles containedin a lower magnetic layer, having an average length in the long axis of0.25 microns or more are used, noise at the short wave length region isincreased, S/N is widely decreased, the surface property of the magneticlayer is deteriorated, and also Y-S as well as Y-S/N are decreased.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a magnetic recordingmedium for a video tape in which output as well as S/N are high at bothshort and long wave length regions, and the magnetic reprintcharacteristic is excellent.

To achieve the foregoing and other objects, the present inventionprovides a magnetic recording medium comprising a nonmagnetic supporthaving thereon plural magnetic layers comprising ferromagnetic particlesdispersed in a binder, said ferromagnetic particles consisting of aniron oxide or a cobalt-containing iron oxide, said plural magneticlayers comprising at least an upper magnetic layer and a lower magneticlayer, wherein said upper magnetic layer has a coercive force (Hc) offrom 650 to 1000 Oe, a crystallite size of the ferromagnetic particlescontained in said upper magnetic layer is less than 500 Å as measured bya X-ray diffraction method, an average length in the long axis of theferromagnetic particles contained in said upper magnetic layer is from0.1 microns to less than 0.25 microns as measured by a transmission typeelectromicrosope, said lower magnetic layer has a coercive force (Hc) offrom 0.6 to 1.0 times that of said upper magnetic layer, a crystallitesize of the ferromagnetic particles contained in said lower magneticlayer is larger than that of said upper magnetic layer and is in therange of 500 Å or less, and an average length in the long axis of theferromagnetic particles contained in said lower magnetic layer is largerthan that of said upper magnetic layer and is in the range of more than0.1microns to less than 0.25 microns.

DETAILED DESCRIPTION OF THE INVENTION

In a magnetic recording medium such as a video tape of VHS or beta type,it is preferred that the coercive force (Hc) of an upper magnetic layeris 650 to 1000 Oe, preferably 700 to 950 Oe. If Hc is less than 650 Oe,output and S/N at the long wave length are deteriorated. If Hc is morethan 1000 Oe, output and S/N at the short wave length region areextremely deteriorated. As mentioned above, in the present invention, Hcof the lower magnetic layer is 0.6 to 1.0 times that of the uppermagnetic layer. If Hc of the lower magnetic layer is too small comparedto the above range, the magnetic reprint characteristics are noticeablydeteriorated.

Hc of the lower magnetic layer is 0.6 to 1.0 times, preferably 0.8 to1.0 times that of the upper magnetic layer. If Hc of the lower magneticlayer is 1.1 times or more that of the upper magnetic layer,electromagnetic characteristics at the long wave length region aredeteriorated. If Hc of the lower magnetic layer is 0.5 times or less,the magnetic reprint characteristic is undesirably reduced. Particularlypreferred Hc of the lower magnetic layer is 600 to 850 Oe.

It has been found that a crystallite size and an average length in thelong axis of ferromagnetic particles widely influence output and S/N atthe short and long wave length regions, and the magnetic reprintcharacteristics.

Namely, a crystallite size of the ferromagnetic particles contained inthe upper magnetic layer is less than 500 Å, preferably from 200 to 400Å and more preferably 300 to 360 ° A. If the crystallite size is 500 Aor more, output as well as S/N at both long and short wave lengthregions is decreased. As seen from the above, if a crystallite size ofthe ferromagnetic particles contained in the upper magnetic layer is toolarge, the crystallite size influences not only output and S/N at thelong wave length region but also output and S/N at the short wave lengthregion. It is preferred that a crystallite size of the ferromagneticparticles contained in the lower magnetic layer is 500 Å or less,preferably 300 Å to 480 ° and more preferably 400 to 450 Å, and also islarger than that of the ferromagnetic particles contained in the uppermagnetic layer.

If a crystallite size of the ferromagnetic particles contained in thelower magnetic layer is too small, output as well as S/N at both longand short wave length regions decrease, because dispersion propertiesdecrease, and surface properties deteriorate.

An average length in the long axis of the ferromagnetic particlescontained in the upper magnetic layer is 0.1 or more to less than 0.25microns and preferably 0.1 to 0.2 microns. If an average length in thelong axis in the upper magnetic layer is less than 0.1 microns, outputand S/N at the long wave length region decrease, because orientationproperties deteriorate, and Br (residual magnetic flux density)decreases. If the average length in the long axis in the upper magneticlayer is 0.25 microns or more, noise at the long wave length regionincrease, and S/N tends to decrease.

It is preferred that an average length in the long axis of theferromagnetic particles contained in the lower magnetic layer is largerthan that of the upper magnetic layer in the range of more than 0.1 toless than 0.25 microns and particularly 0.15 to 0.24 microns. If anaverage length in the long axis in the lower magnetic layer is 0.1microns or less, the magnetic reprint characteristics deteriorate. If anaverage length in the long axis in the lower magnetic layer is 0.25microns or more, output and S/N at the short wave length region arenoticeably liable to deteriorate, and output and S/N at the long wavelength region are a1so liable to deteriorate.

In the present invention, when Hc of the upper magnetic layer is made tobe 650 to 1000 Oe, output as well as S/N at both long and short wavelength regions is improved. When Hc of the lower magnetic layer is madeto be 0.6 to 1.0 times that of the upper magnetic layer, output and S/Nat the short wave length region are increased, and the magnetic reprintcharacteristics are also improved. When a crystallite size of theferromagnetic particles contained in the upper magnetic layer is made tobe less than 500 Å, S/N at both long and short wave length regions isimproved. When a crystallite size of the ferromagnetic particles in thelower magnetic layer is made to be larger than that of the uppermagnetic layer in the range of 500 Å or less, surface properties areincreased and output as well as S/N at both long and short wave lengthregions is noticeably improved. When an average length in the long axisof the ferromagnetic particles in the upper magnetic layer is made to be0.1 or more to less than 0.25 microns, output as well as S/N at bothlong and short wave length regions is extremely improved. Further, whenan average length in the long axis of the ferromagnetic particles in thelower magnetic layer is made to be larger than that of the uppermagnetic layer in the range of more

output as well than 0.1 microns to less than 0.25 microns as S/N at theshort wave length region are noticeably improved. It is quite unexpectedthat output as well as S/N at both long and short wave length regions,and the magnetic reprint characteristics are improved by using acombination of a specified range of components in the present invention.

In the present invention, a crystallite size of ferromagnetic particlesis measured from (1,1,0) plane by X-ray diffraction, and calculated fromthe formula of Hall. An average length in the long axis of ferromagneticparticles is measured by transmission electron microscope.

Ferromagnetic particles which can be used in the present inventioninclude conventional one such as an iron oxide [e.g., α-Fe₂ O.sub. 3,Fe₃ O₄, and FeOx (x=1.33 to 1.50)] or a cobalt-containing iron oxide[e.g., α-Fe₂ O₃, Fe₃ O₄, and FeOx (x=1.33 to 1.50)]. Thecobalt-containing iron oxide includes a Co-modified iron oxide such as aCo-doped iron oxide or a Co-dissolved iron oxide, of a Co-coated ironoxide, etc. Particularly preferred is Co-γ-Fe₂ O₃ which is used in bothupper and lower layers.

In the general manufacture of an acicular iron oxide, alkali such NaOhetc., is added to an aqueous solution of ferrous sulfate to adjust pH to10 or more while heated to 30° C. followed by blowing air into thereaction system. The oxidation is continued to form α-FeOOH, and thecrystallite size is varied by alteration in pH. Namely, if the pH isincreased, the crystallite size of α-FeOOH is increased. The crystallitesize of the ferromagnetic particles which can be used in the presentinvention can be controlled by treating the above α-FeOOH in asubsequent step. (Regarding γ-iron oxide, it can also be manufacturedusing conventional methods).

In the present invention, a binder used for forming a magnetic layer ora backing layer which is formed if desired, can be a thermoplasticresin, a thermosetting resin, a reactive type resin or a mixturethereof, which are conventionally known.

Thermoplastic resins which can be used as a binder include resins havinga softening temperature of 150° C. or less, an average molecular weightof 10,000 to 300, 000 and a degree of polymerization of about 50 to2,000, such as, for example, a copolymer of vinyl chloride and vinylacetate, a copolymer of vinyl chloride, a copolymer of vinyl chlorideand vinylidene chloride, a copolymer of vinyl chloride andacrylonitrile, a copolymer of acrylic acid ester and acrylonitrile, acopolymer of acrylic acid ester and vinylidene chloride, a copolymer ofacrylic acid ester and styrene, a copolymer of methacrylic acid esterand acrylonitrile, a copolymer of methacrylic acid ester and vinylidenechloride, a copolymer of methacrylic acid ester and styrene, a urethaneelastomer, a nylon-silicon resin, nitrocellulose-polyamide resins,polyvinyl fluoride, a copolymer of vinylidene chloride andacrylonitrile, a copolymer of butadiene and acrylonitrile, a polyamideresin, a polyvinyl butyral, cellulose derivatives (e.g., celluloseacetic butyrate, cellulose diacetate, cellulose triacetate, cellulosepropionate, nitrocellulose, ethylcellulose, methylcellulose,propylcellulose, methylethylcellulose, carboxymethylcellulose,acetylcellulose), a copolymer of styrene and butadiene, a polyesterresin, a copolymer of chlorovinyl ether and acrylic acid ester, an aminoresin, various synthetic rubber thermoplastic resins or a mixture of twoor more of any of the above resins.

Thermosetting or reactive type resins which can be used as a binderinclude resins having a molecular weight of 200,000 or less when theyare in the coating composition. When the coating composition for forminga magnetic layer is coated, dried and then heated, these resins reactand the molecular weight thereof becomes infinite due to such reactionssuch as a condensation reaction, an adduct reaction and the like. Amongthese resins, resins that do not soften nor melt until they areheat-decomposed are preferred. Specifically, the resins include phenolicresins, phenoxy resins, epoxy resins, hardenable polyurethane resins,urea resins, melamine resins, alkyd resins, silicon resins, reactiveacrylic resin, epoxy polyamide resins, nitrocellulose melamine resins, amixture of high molecular weight polyester resins and isocyanateprepolymer, a mixture of a copolymer of methacrylate and a diisocyanateprepolymer, a mixture of a polyester polyol and a polyisocyanate, a ureaand formaldehyde resin, a mixture of a low molecular weight glycol/ahigh molecular weight diol/triphenylmethane triisocyanate, polyamineresins, polyimine resins, or a mixture of two or more of any of theabove resins.

One or two or more of the above resins can be used as a binder forforming a magnetic layer.

In addition to the above binder, additives can be added to the binder.Regarding the compounding ratio of the ferromagnetic particles and thebinder used in the magnetic layer, generally 5 to 300 parts by weight(preferably 10 to 100 parts by weight and more preferably 20 to 40 partsby weight) of the binder can be used per 100 parts by weight of theferromagnetic particles.

Regarding the compounding ratio of nonmagnetic particles and the binderused in the backing layer, generally 30 to 300 parts by weight(preferably 40 to 150 parts by weight) of the binder can be used per 100parts by weight of the nonmagnetic particles.

Additives which can be added to the binder include dispersing agents,lubricating agents, abrasive agents, antistatic agents, anti-oxidizingagents, solvents or the like.

It is preferred that the thermoplastic resin, thermosetting resin, andreactive type resin further contain generally from 1 to 6 kinds offollowing functional groups in addition to those contained in the mainchain; carboxylic acid, sulfinic acid, sulfenic acid, sulfonic acid,phosphoric acid, sulfuric acid, phosphonic acid, phosphine, boric acid,acid groups such as a sulfuric acid ester group or a phosphoric acidester group or an alkyl ester group of these ester groups (such acidgroups may be sodium salts of them), amino acids, amino sulfonic acids,sulfuric acid esters or phosphoric acid esters of aminoalcohol,amphoteric groups such as alkyl betain agents, amino groups, iminogroups, imide groups, amide groups, epoxy groups, hydroxyl groups,alkoxy groups, thiol groups, halogen groups, silyl groups, and siloxanegroups, in an amount of 1×10⁻⁶ to 1×10⁻² equivalent per lg of the resin.

Polyisocyanates which can be used in the present invention include, forexample, tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate,hexamethylene diisocyanate, xylylene diisocyanate,naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophoronediisocyanate or triphenylmethane triisocyanate, the reaction products ofthese isocyanates and polyalcohols, and polyisocyanates having 2 to 15monomeric units produced by condensing these isocyanates. Suchpolyisocyanates have an average molecular weight of preferably 100 to20,000.

The above-described polyisocyanates are commercially available under thetrade names of "Coronate L", "Coronate HL", "Coronate 2023", "Coronate2031", "Millionate MR", and "Millionate MTL" produced by NipponPolyurethane Co., Ltd.; "Takenate D-102", "Takenate D-110N", "TakenateD-200", "Takenate D-202", "Takenate 300S", "Takenate 500", produced byTakeda Chemical Industries, Ltd.; and "Sumidule T-80", "Sumidule 44S","Sumidule PF", "Sumidule L", "Sumidule N", "Desmodule L", "DesmoduleIL", "Desmodule N", "Desmodule HL", "Desmodule T65", "Desmodule 15","Desmodule R", "Desmodule RF", "Desmodule SL", "Desmodule Z4273",produced by Sumitomo Bayer Co., Ltd. These polyisocyanates can be usedalone or in mixture by taking advantage of the differences in theirhardening reactivities.

In order to accelerate curing reactions, there can be used compoundshaving hydroxyl groups (e.g., butanediol, hexanediol, polyurethaneshaving a molecular weight of 1000 to 10,000, or water), compounds havingamino groups (e.g., monomethylamine, dimethylamine, or trimethylamine)or catalysts of metal oxides in addition to the polyisocyanates. It ispreferred that the compounds having such hydroxyl groups or amino groupsbe polyfunctional. It is preferred to use 5 to 40% (particularly 20 to40%) by weight of the polyisocyanates based on the total weight of thebinder.

Dispersing agents which can be used in the present invention includefatty acids R₁ COOH (in which R₁ is an alkyl group having 9 to 25 carbonatoms) having 10 to 26 carbon atoms such as capric acid, caproic acid,lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid,oleic acid, elaidic acid, linoleic acid, linolenic acid or stearol acid;alkali metals (e.g., Li, Na, K, NH₄ ⁺) or alkali earth metals (e.g., Mg,Ca, Ba) of these fatty acids; soaps of metals such as copper or lead;fatty acid amides of these fatty acids; and lecithin. Moreover, higheralcohols having 4 or more carbon atoms (e.g., butanol, octyl alcohol,myristyl alcohol, stearyl alcohol), their sulfuric acid esters, theirphosphoric acid esters, and their amine compounds can be used asdispersing agents. Further, there can be used as dispersing agentspolyalkyleneoxides; their sulfuric acids esters, their phosphoric acidesters, their amine compounds; and sulfosuccinic acids, sulfosuccinicacid esters, etc. In order to change the compatibility with a binder orthe characteristics of it, substituent groups such as Si or F can beintroduced to these compounds. These dispersing agents are usually usedin one or more kinds. One kind of the dispersing agents are added in anamount of generally 0.005 to 20 parts by weight per 100 parts by weightof the binder. The dispersing agents may be previously applied on thesurfaces of ferromagnetic particles and nonmagnetic particles, or may beadded in the course of the dispersion process. In addition to the abovedispersing agents, there can be preferably used surface active agentssuch as carboxylic acids or phosphoric acid esters etc., and fluorinecontaining surface active agents such as "Fluorad FC 95", "FC 129", "FC430", and "FC 431".

Lubricating agents which can be used for forming a magnetic layer or abacking layer include inorganic fine particles such as molybdenumdisulfide, boron nitride, fluorinated graphite, calcium carbonate,barium sulfate, silicon oxides, titanium oxides, zinc oxides, stannousoxide, or tungsten disulfide; acryl styrene resin fine particles,benzoguanamine resin fine particles, melamine resin fine particles,polyolefin resin fine particles, polyester resin fine particles,polyamide resin fine particles, polyimide resin fine particles, orpolyethylene fluoride resin fine particles; silicone oils fatty acidmodified silicone oils, graphite, fluorinated alcohols, polyolefins(e.g., polyethylene waxes), polyglycols (e.g., polyethylene oxidewaxes), tetrafluoroethylene oxide waxes, polytetrafluoroglycols,perfluorofatty acids, perfluoro fatty acid esters, perfluoro alkylsulfuric acid esters, perfluoro alkyl phosphoric acid esters, alkylphosphoric acid esters, polyphenyl ethers; fatty acids esters ofmonobasic fatty acids having 10 to 20 carbon atoms and alcoholscontaining at least one kind of monovalent, divalent, trivalent,tetravalent, or hexavalent alcohols having 3 to 12 carbon atoms; fattyacid esters having total carbon atoms of 11 to 28 and monobasic fattyacids having 10 or more carbon atoms and monovalent through hexavalentalcohols. Fatty acids having 8 to 22 carbon atoms, fatty acid amides oraliphatic alcohols can also be used as organic lubricating agents.

Examples of such organic lubricating agents include butyl caprylate,octyl caprylate, ethyl laurate, butyl laurate, octyl laurate, ethylmyristate, butyl myristate, octyl myristate, ethyl palmitate, butylpalmitate, octyl palmitate, ethyl stearate, butyl stearate, octylstearate, amyl stearate, anhydrosorbitan monostearate, anhydrosorbitandistearate, anhydrosorbitan tristearate, anhydrosorbitan tetrastearate,anhydrosorbitanethyleneoxide monostearate, oleyl oleate, oleyl alcohol,and lauryl alcohol. These organic lubricating agents can be used aloneor in combination. Moreover, additives for lubricating oils may be usedwith the lubricating agent of the present invention. Such additivesinclude antioxidizing agents (e.g., alkyl phenols), rust-preventingagents (e.g., naphthenic acid, alkenyl succinic acid, dilaurylphosphate), oiling agents (e.g., rape seed oil, lauryl alcohol),high-pressure lubricating agents (e.g., dibenzylsulfide, tricresylphosphate, tributyl phosphite), detergent dispersing agents, viscosityindex improvers, pour point depressants, and defoaming agents. Thelubricating agents are added in an amount of generally 0.05 to 20 partsby weight, per 100 parts by weight of the binder.

The abrasive agents for forming the magnetic layer or the backing layerin the present invention include α-alumina, γ-alumina, α-γ-alumina,fused alumina, silicon carbide, chromium oxide, cerium oxide, corundum,diamond, α-iron oxide, garnet, emery (main components: corundum andmagnetite), silica rock, silicon nitride, boron nitride, molybdenumcarbide, boron carbide, tungsten carbide, titanium carbide, quartz,tripoli, diatomite, or dolomite, etc. Such abrasive agents have a Moh'shardness of 6 or higher, preferably 8 or higher, and can be used aloneor in combination up to 4 kinds of them. The abrasive agents have anaverage particle diameter of generally 0.005 to 5 microns, preferably0.01 to 2 microns. The amount of abrasive agents is generally 0.01 to 20parts by weight and preferably 1 to 15 parts by weight, per 100 parts byweight of the binder.

Organic solvents which can be used in arbitrary proportions at steps ofdispersing, mixing, coating in the present invention include ketonesolvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone,cyclohexanone, isophorone or tetrahydrofuran; alcohol solvents such asmethanol, ethanol, propanol, butanol, isobutyl alcohol, isopropylalcohol or methyl hexanol; ester solvents such as methyl acetate, ethylacetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyllactate, or glycol acetate monoethyl ether; ether solvents such asdiethyl ether, tetrahydrofuran, glycol dimethyl ether, glycol monoethylether, or dioxane; aromatic hydrocarbon solvents such as benzene,toluene, xylene, cresol, chlorobenzene or styrene; chlorinatedhydrocarbon solvents such as methylene chloride, ethylene chloride,carbon tetrachloride, chloroform, ethylene chlorohydrin ordichlorobenzene; N,N-dimethyl formamide; and hexane.

The method for mixing and kneading is not particularly limited, and theorder of adding each component can optionally be determined. The mixingand kneading device to prepare a coating composition for a magneticlayer and a coating composition for a backing layer can be aconventional one, such as, for example, a two-roll mill, a three-rollmill, a ball mill, a pebble mill, a Trommel, a sand grinder, a Szegvar,an attritor, a high speed impeller, a dispersing machine, a high speedstone mill, a high speed impact mill, a Disper, a kneader, a high speedmixer, a ribbon blender, a co-kneader, an intensive mixer, a tumbler, ablender, a disperser, a homogenizer, a single axial screw extruder, abiaxial screw extruder, or an ultrasonic dispersing device. Details oftechniques relating to mixing, kneading and dispersing are disclosed inT.C. Patton, Paint Flow and Pigment Dispersion John Wiley & SonsCompany, 1964; Shinichi Tanaka, Industrial Materials Vol. 25, 37(1977)etc., and reference in these papers. In these papers, theabove-mentioned devices are optionally combined to forward the coatingcomposition, followed by coating. Such techniques are also disclosedmethods described in the papers and reference above, mixing, kneading,and dispersing are also carried out in the present invention to preparea coating composition for a magnetic layer and a coating composition fora backing layer.

The components constituting the coating composition are dissolved in anorganic solvent to form a coating solution. The coating solution iscoated on a support and then dried. When the support is used as a film,it has a thickness of generally about 2.5 to 100 microns, preferably 3to 70 microns. When the support is used as a disk or a card, it has athickness of generally about 0.03 to 10 mm. When the support is used asa drum, it may be a cylindrical shape.

The film can be made of plastics, such as polyester (e.g., polyethyleneterephthalate, polyethylene naphthalate), polyolefins (e.g.,polypropylene, polyethylene), cellulose derivatives (e.g., cellulosetriacetate or cellulose diacetate), vinyl resins (e.g., polyvinylchloride, polyvinylidene chloride), polycarbonate, polyamide resins orpolysulfone, metal materials such as aluminum or copper, and ceramicssuch as glass. These supports may be subjected to pre-treatment such ascorona-discharge treatment, plasma treatment, undercoating treatment,heating treatment, dust removal treatment, metal vapor depositiontreatment or alkaline treatment.

The coating composition for forming the magnetic layer and the backinglayer can be coated on a support by various coating methods such as anair doctor coating, a blade coating, an air knife coating, a squeezecoating, a dipcoating, a reverse roll coating, transfer roll coating, agravure coating, a kiss coating, a cast coating, a spray coating, a barcoating, or a spin coating, etc. These coating methods are concretelydescribed in Coating Industry by Asakura Company, pages 253 to 277,(March 20, 1971). In the present invention, a "wet-on-wet coatingmethod" is preferably adopted, where a coating composition for forminginner magnetic layers (i.e., lower layers) and a coating composition forforming a surface magnetic layer (i.e., upper layers) are coated on asupport while the compositions for their magnetic layers are wet.

The above "wet-on-wet coating method" is described in JP-A-61-139929.

A method for dispersing the ferromagnetic particles in a binder and amethod for coating them on a support and disclosed in detail inJP-A-54-46011 and JP-A-54-21805.

The magnetic layers thus provided on the support preferably aresubjected to magnetic orientation to orientate the ferromagneticparticles while they are being dried, and then the magnetic layers aredried. The support is transferred generally at a rate of 10 to 1000m/minute and is dried at a temperature of 20 to 130° C.

If desired, it is subjected to a surface smoothing treatment asdescribed in U.S. Pat. No. 3,473,960 and then slit to a desired shape.It is preferred that a filler is continuously subjected to steps ofsurface treating, mixing, dispersing, coating, heat-treating,calendering, irradiating, surface polishing, and cutting. These stepsmay be separated into some groups. In these steps, temperature andhumidity can be controlled. Namely, the temperature is generally 10 to130° C., and the humidity is generally 5 to 20 mg/m³ as measured bywater content in air.

These steps are disclosed in, for example, JP-B-40-23625 (The term"JP-B" as used herein means an "examined Japanese patent publication"),JP-B-39-28368, and U.S. Pat. No. 3,473,960. Also, JP-B-13181 shows basicand important techniques in this field.

The present invention will be illustrated in more detail by thefollowing Example. In Example, all parts are by weight.

EXAMPLE

    ______________________________________                                        Co--FeOx (x = 1.45, Regarding Hc, Crystallite size,                                                      100    parts                                       and Average length in the long axis, see                                      Table 1)                                                                      Copolymer of vinyl chloride and vinyl acetate                                                            15     parts                                       (Amount of sulfonic acid group: 0.25 wt. %,                                   Degree of polymerization: 400)                                                Polyester polyurethane     5      parts                                       (Amount of sulfonic acid group: 0.1 wt. %)                                    Polyisocyanate ("Coronate L")                                                                            6.7    parts                                       Electroconductive carbon   1      part                                        (Particle diameter: 10 microns)                                               Oleic acid                 2      parts                                       Butyl acetate              20     parts                                       Methyl ethyl ketone (MEK)  80     parts                                       ______________________________________                                    

The coating solution having the above composition was coated on apolyethylene terephthalate (PET) film having a thickness of 14 micronsto form a single magnetic layer or plural magnetic layers. The singlelayer was 4 microns thick. The plural layers comprised an upper layerhaving a thickness of 0.5 microns and a lower layer having a thicknessof 3.5 microns.

The thus obtained samples were evaluated in the following manner and theresults are shown in Table 1.

Y-S (Electromagnetic characteristics at the long wave length region):

Video signals of 50% white were recorded by a standard recording imagecurrent. The average envelope of the reproduced output was measuredusing an oscilloscope and calculated from the following formula (1).

    Sensibility of reproduced output (dB)=20log.sub.10 V/Vo    (1)

(V: an average value, V_(o) : the average value of comparative example12-1).

C-S (Electromagnetic characteristics at the short wave length region):

Video signals of one color were recorded by a standard recording imagecurrent, and calculated according to the same method as above.

Y-S/N :

The video S/N (signal/noise) was measured using a noise meter ("925R"produced by Shibasoku Co., Ltd.). The results are shown in Table 1 inrelative values as compared to a tape of "comparative example 12-1"which was used as a standard tape. The noise levels of the tapes weremeasured with a video tape recorder "NV-8200" produced by MatsushitaElectric Industrial Co., Ltd., and the noise meter as described aboveusing a low-pass filter (4.2 MHz) and a high-pass filter (10 KHz).

C-S/N :

The same procedure as in Y-S/N was repeated, except that a low-passfilter (500 KHz AM) was used.

With an input level which is higher than a standard input level by 10dB, sine wave signals at 1 KHz were recorded with no signal, 1 KHz, nosignal (The reel rotates about 10 times), 1 KHz (The reel rotates onceor less), and no signal, in that order.

This recording was repeated several times, followed by leaving 48 hoursat a temperature of 30±0.5° C. The reproduced output level was measuredand calculated by the following formula. Also, the maximum magneticreprint signal level of no signal part passed through a band-passfilterat 1 kHz was measured and calculated from the following formula.

    Magnetic reprint (dB)=A-B

[A: a reproduced output level (dBs) at 1 KHz, B: a maximum magneticreprintsignal level (dBs)]

                                      TABLE 1                                     __________________________________________________________________________            Upper Magnetic Layer                                                                         Lower Magnetic Layer                                          Ferromagnetic Particles                                                                      Ferromagnetic Particles                                            Crystallite                                                                         Length in                                                                              Crystallite                                                                         Length in                                            Hc  size  the long                                                                           Hc  size  the long                                                                           Y-S                                                                              Y-S/N                                                                             C-S                                                                              C-S/N                                                                             S/P                               (θe)                                                                        (Å)                                                                             axis (μ)                                                                        (θe)                                                                        (Å)                                                                             axis (μ)                                                                        (dB)                                                                             (dB)                                                                              (dB)                                                                             (dB)                                                                              (dB)                       __________________________________________________________________________    Comparative                                                                          630 360   0.2  650 450   0.24 +2.5                                                                             +2.8                                                                              +3.2                                                                             +3.5                                                                              53                         Example 1                                                                     Example 1-2                                                                          650 "     "    "   "     "    +3.0                                                                             +3.5                                                                              +3.2                                                                             +3.5                                                                              54                         Example 1-1                                                                          700 "     "    "   "     "    +3.2                                                                             +3.7                                                                              +3.1                                                                             +3.4                                                                              53                         Example 1-3                                                                          850 "     "    "   "     "    +3.4                                                                             +3.8                                                                              +3.0                                                                             +3.3                                                                              54                         Example 1-4                                                                          1000                                                                              "     "    "   "     "    +3.5                                                                             +3.9                                                                              +2.9                                                                             +3.2                                                                              55                         Comparative                                                                          1020                                                                              "     "    "   "     "    +3.5                                                                             +3.9                                                                              +2.5                                                                             +2.8                                                                              55                         Example 2                                                                     Comparative                                                                          700 "     "    400 "     "    +3.2                                                                             +3.6                                                                              +3.3                                                                             +3.6                                                                              47                         Example 3                                                                     Example 2-2                                                                          "   "     "    420 "     "    +3.3                                                                             +3.7                                                                              +3.2                                                                             +3.5                                                                              51                         Example 2-3                                                                          "   "     "    500 "     "    +3.2                                                                             +3.6                                                                              +3.1                                                                             +3.4                                                                              52                         Example 2-1                                                                          "   "     "    650 "     "    +3.2                                                                             +3.7                                                                              +3.1                                                                             +3.4                                                                              53                         Example 2-4                                                                          "   "     "    700 "     "    +3.3                                                                             +3.8                                                                              +2.9                                                                             +3.2                                                                              54                         Comparative                                                                          700 360   0.2  720 450   0.24 +3.2                                                                             +3.7                                                                              +2.5                                                                             +2.7                                                                              55                         Example 4                                                                     Example 3-2                                                                          "   200   "    650 "     "    +3.5                                                                             +3.9                                                                              +3.3                                                                             +3.6                                                                              53                         Example 3-3                                                                          "   300   "    "   "     "    +3.3                                                                             +3.8                                                                              +3.2                                                                             +3.5                                                                              53                         Example 3-1                                                                          "   360   "    "   "     "    + 3.2                                                                            +3.7                                                                              +3.1                                                                             +3.4                                                                              53                         Example 3-4                                                                          "   440   "    "   "     "    +3.0                                                                             +3.5                                                                              +3.0                                                                             +3.3                                                                              54                         Comparative                                                                          "   520   "    "   "     "    +2.9                                                                             +2.9                                                                              +2.8                                                                             +3.0                                                                              54                         Example 5                                                                     Comparative                                                                          "   360   "    "   300   "    +2.8                                                                             +3.3                                                                              +2.8                                                                             +2.9                                                                              51                         Example 6                                                                     Example 4-1                                                                          "   "     "    "   400   "    +3.2                                                                             +3.7                                                                              +3.1                                                                             +3.4                                                                              53                         Example 4-2                                                                          "   "     "    "   450   "    +3.1                                                                             +3.6                                                                              +3.0                                                                             +3.3                                                                              53                         Example 4-3                                                                          "   "     "    "   500   "    +3.0                                                                             +3.5                                                                              +2.7                                                                             +2.9                                                                              54                         Comparative                                                                          700 360   0.2  650 550   0.24 +2.8                                                                             +3.3                                                                              +2.5                                                                             +2.6                                                                              55                         Example 7                                                                     Comparative                                                                          "   "      0.08                                                                              "   450   "    +2.6                                                                             +3.0                                                                              +3.0                                                                             +3.1                                                                              53                         Example 8                                                                     Example 5-2                                                                          "   "     0.1  "   "     "    +3.0                                                                             +3.5                                                                              + 3.0                                                                            +3.3                                                                              54                         Example 5-1                                                                          "   "     0.2  "   "     "    +3.2                                                                             +3.7                                                                              +3.1                                                                             +3.4                                                                              53                         Example 5-3                                                                          "   "      0.22                                                                              "   "     "    +3.0                                                                             +3.5                                                                              +3.0                                                                             +3.3                                                                              53                         Comparative                                                                          "   "      0.28                                                                              "   "     "    +2.9                                                                             +3.0                                                                              +2.8                                                                             +3.0                                                                              53                         Example 9                                                                     Comparative                                                                          "   "     0.2  "   "     0.15 +3.0                                                                             +3.3                                                                              +3.0                                                                             +3.3                                                                              49                         Example 10                                                                    Example 6-2                                                                          "   "     "    "   "     0.22 +3.3                                                                             +3.8                                                                              +3.2                                                                             +3.6                                                                              53                         Example 6-1                                                                          "   "     "    "   "     0.24 +3.2                                                                             +3.7                                                                              +3.1                                                                             +3.4                                                                              53                         Comparative                                                                          "   "     "    "   "     0.28 +3.0                                                                             +3.3                                                                              +3.0                                                                             +2.8                                                                              55                         Example 11                                                                    Single layer                                                                  Comparative                                                                          700 360   0.2  --  --    --    0  0   0  0  53                         Example 12-1                                                                  Comparative                                                                          650 450    0.25                                                                              --  --    --   -1.0                                                                             -2.0                                                                              -1.0                                                                             -2.5                                                                              57                         Example 12-2                                                                  __________________________________________________________________________

As is apparent from results of Table 1, the sensitivity and output atthe long wave length region are mainly influenced by theferromagnetic-particles of the upper magnetic layer. The sensitivity andoutput are liable to be deteriorated when Hc is too small, and acrystallite size as well as an average length in the long axis of theferromagnetic particles are too large. This effect is particularly largewhen Hc of the upper magnetic layer is too large. This is a quiteunexpected fact.

The sensitivity and output at the short wave length region are mainlyinfluenced by ferromagnetic particles of the lower magnetic layer, andthey are noticeably deteriorated when Hc is too large and a crystallitesize as well as an average length in the long axis of the ferromagneticparticles are too large. Contrary to the above, sensitivity and outputarenoticeably improved in the range defined by the present invention.Moreover, the sensitivity and output at the short wave length region areliable to be deteriorated when Hc, a crystallite size, and an averagelength in the long axis of the ferromagnetic particles in the uppermagnetic layer are too large. Contrary to the above, they are improvedin the range defined by the present invention. This is also a quiteunexpected fact.

Regarding magnetic reprint effects, it has been known that they aredeteriorated when ferromagnetic particles are made small. Magneticreprinteffects are also deteriorated when a crystallite size and anaverage lengthin the long axis of the ferromagnetic particles in bothupper and lower magnetic layers are made too small. However, it is quiteunexpected that magnetic reprint effects are greatly influenced when Hcof the lower magnetic layer is too small. This shows prominent effectsattained by the present invention.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A magnetic recording medium comprising anonmagnetic support having thereon plural magnetic layers comprisingferromagnetic particles dispersed in a binder, said ferromagneticparticles consisting of an iron oxide or a cobalt-containing iron oxide,said plural magnetic layers comprising at least an upper magnetic layerand a lower magnetic layer, wherein said upper magnetic layer has acoercive force (Hc) of from 650 to 1000 Oe, a crystallite size of theferromagnetic particles contained in said upper magnetic layer is lessthan 500 Å as measured by a X-ray diffraction method, an average lengthin the long axis of the ferromagnetic particles contained in said uppermagnetic layer is from 0.1 microns to less than 0.25 microns as measuredby a transmission type electron microscope, said lower magnetic layerhas a coercive force (Hc) of from 0.6 to 1.0 times that of said uppermagnetic layer, a crystallite size of the ferromagnetic particlescontained in said lower magnetic layer is larger than that of said uppermagnetic layer and is in the range of 500 Å or less, and an averagelength in the long axis of the ferromagnetic particles contained in saidlower magnetic layer is larger than that of said upper magnetic layerand is in the range of from more than 0.1 microns to less than 0.25microns.
 2. The magnetic recording medium as in claim 1, wherein thecoercive force (Hc) of said lower magnetic layer is from 0.8 to 1.0times that of the upper magnetic layer.
 3. The magnetic recording mediumas in claim 1, wherein the crystallite size of the ferromagneticparticles contained in the upper magnetic layer is from 200 Å to 400 Å.4. The magnetic recording medium as in claim 1, wherein the crystallitesize of the ferromagnetic particles contained in the lower magneticlayer is from 300 A to 480 Å.
 5. The magnetic recording medium as inclaim 1, wherein the ferromagnetic particles comprise an iron oxideselected from the group consisting of α-Fe₂ O₃, Fe₃ O₄, and FeOx (x=1.33to 1.50).
 6. The magnetic recording medium as in claim 1, wherein theiron oxide of the cobalt-containing iron oxide comprises Co-α-Fe₂ O₃,Co-Fe₃ O₄, and Co-FeOx (x=1.33 to 1.50).
 7. The magnetic recordingmedium as in claim 1, wherein the ferromagnetic particles compriseCo-γ-Fe₂ O₃.